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This paper describes some new results on the day‐to‐day behavior of the location of the equatorial anomaly in the columnar electron content. The measurements were made from a unique network of stations covering dip latitudes from 0°N to 25°N during the period in 1975–1976 near sunspot minimum when the ATS 6 satellite was visible from India. The latitudinal distribution of the total electron content on a particular day was found to depend only on the strength of the electrojet current, as determined by the difference of the horizontal magnetic field at stations on the magnetic equator and another outside the equatorial electrojet, rather than on the horizontal magnetic field measured only at the equatorial station. During magnetically disturbed periods the presence or absence of the anomaly was also found to depend only upon the strength of the electrojet current. The varying electrojet current, that is, the equatorial electric field, is very effective in moving the ionization to the anomaly latitudes of 15°–20°, but the columnar electron content over the magnetic equator remains relatively constant.

Ionospheric total electron content (TEC) measurements, obtained simultaneously at several locations, can be processed using computerized tomography (CT) algorithms to obtain two-dimensional images of ionospheric electron density. Using TEC data, computerized ionospheric tomography (CIT) reconstructs an image of the electron density structures in a vertical slice above the receiving stations. We successfully applied this technique to realistic simulations of ionospheric density variations over 16 ø of latitude and a height range of 50 to 1000 km. A method for approximating the peak height and scale height of the electron density profile will be discussed as well as a reconstruction technique based on the multiplicative algebraic reconstruction technique algorithm and a back projection based initial guess. The quality of reconstructions is considered for two geometries and image resolutions. In particular, the image of a mid-latitude trough with background horizontal density gradient and large-scale irregular structures has been reconstructed from TEC data generated from a model based on an incoherent scatter radar observation. The CT reconstructed image was compared with the original contour map obtained by the incoherent scatter radar. Good agreement has been achieved. The CIT technique has also been applied to a modeled ionosphere to calculate the range rate corrections for a Doppler-tracking radar. INTRODUCTIONRecently, Austen et al. [1988] have demonstrated the feasibility of using a computer tomography (CT) technique for imaging ionospheric electron density. The CT technique involves using one-dimensional information to reconstruct a two-dimensional image. The data consist of measurements of the line integral of electron density in the ionosphere for many different paths. The line integral paths can be considered as many unique rays traversing the plane of the image. The image region can be gridded into small areas or pixels, and within each pixel the parameter will assume some constant value. What is desired is the distribution of the parameter over the image. In the case of the ionosphere the integrated parameter is electron density. The line integral of electron density over some path is the total electron content (TEC). Using a chain of stations receiving the beacon signal from an orbiting satellite, a geometry can be created such that the TEC measurements can be used to reconstruct a vertical slice image, or vertical cross section, of ionospheric electron density in a vertical plane between the satellite and the ground stations.The feasibility of using CT for imaging ionospheric electron density has been shown by Austen et al. [1988] by applying the technique to successfully reconstruct iononspheric structures from synthetic TEC data generated from computer models. In this paper we will discuss the results from a further test of the technique. From a mapping of the mid-latitude ionospheric trough obtained by the Chatanika radar [Weber et al., 1985] the Air Force Geophysics Laboratory (AFGL) group (J. A...

Routine Faraday rotation observations of the VHF signal from the geostationary satellite ATS 3 made at Sagamore Hill (Massachusetts) revealed that an unusually large and rapid decay in the ionospheric total electron content (TEC) occurred near 1240 EST on May 14, 1973. The disturbance appeared as a dramatic 'bite-out' of substantial magnitude (>_50%) and duration (of the order of hours) in the expected diurnal TEC curve for that day. Observations from other sites revealed that a 'hole' in the ionospheric F region was created over a region approximately 1000 km in radius. The onset of the TEC disturbance occurred within 10 min of the launch of NASA's Skylab workshop by a Saturn 5 rocket. As the rocket moved at F regio_n -heights, the burning second-stage engines passed within 150 km of the Sagamore Hill ray path to ATS 3. A detailed analysis of the aeronomic reactions initiated by the constituents of the exhaust field revealed that the F•. region plasma experienced a devastating loss process as the plume expanded. The specific mechanism involved was the rapid ion-atom interchange reactions between the ionospheric O + and the hydrogen and water vapor molecules in the plume, followed by dissociative recombination of the molecular ions. Model calculations of the diffusion of the plume in the ionosphere and its effect upon continuity equation calculations for TEC showed an excellent agreement with the observed onset and magnitude of the effect. The phenomenon has interesting astrophysical and geophysical implications.

Coordinated measurements of F region plasma patches were conducted on February 3/4, 1984, from Thule and Sondrestrom, Greenland. Optical, ionosonde, amplitude scintillation, total electron content (TEC), and incoherent scatter radar measurements were combined to reveal several new aspects of the structure and transport of these localized regions of enhanced F region ionization. For the first time these patches were directly tracked flowing in the antisunward direction over distances of 3000 km from the center of the polar cap to the poleward edge of the auroral oval. Quantitative measurements of TEC show increases of 10–15 TEC units within the patches, above a background polar cap value of 5 TEC units. Amplitude scintillation measurements show the presence of ionospheric irregularities through the entire patch, with a weak indication of stronger scintillation on the trailing (or E × B unstable) edge.